Rhus garwellii is known from specimens which are recovered from an outcrop of the early Eocene, Ypresian[1][2] Klondike Mountain Formation in Republic.

[2] The plant community preserved in the Klondike Mountain formation is a mixed conifer–broadleaf forest with large pollen elements of birch and golden larch, but also having notable traces of fir, spruce, cypress, and palm.

[1] The etymology of the chosen specific name garwellii is a patronym honoring Gar Rothwell recognizing his excitement and dedication for paleobotany.

[1] In addition to Rhus garwellii Flynn et al also provided descriptions of the coeval species R. boothillensis and R. republicensis and a gave a redescription of R. malloryi in the 2019 paper.

The teeth along the margin have distinctly pointed tips with convex basal and apical sides running towards angular sinuses.